Method, system and apparatus for improving reception in multiple access communication systems

ABSTRACT

An apparatus, system and method for improving the SNR of a desired signal received at a receiver in a multiple access communication system is disclosed. The apparatus, system and method subtracts known or knowable signals from the total signal received at the receiver and the desired signal is then determined from the result of the subtraction. The known, or knowable, signals can be synchronization signals or other interfering channel signals transmitted by the transmitter, such as a wireless network base station, serving the receiver, such as a subscriber station in such a wireless network, and/or can be such signals transmitted by another transmitter, such as an adjacent base station, or an adjacent sector in multi-sector systems.

FIELD OF THE INVENTION

The present invention relates to a method, apparatus and system forimproving reception in a multiple access communication system. Morespecifically, the present invention provides a method, apparatus andsystem for improving reception in a multiple access communication systemby subtracting known, or knowable, received interference signals fromthe total received signal.

BACKGROUND OF THE INVENTION

Many multiple access communication techniques are known including timedivision multiple access (TDMA), frequency division multiple access(FDMA), orthogonal and vector orthogonal frequency division multiplexing(OFDM or VOFDM), code division multiple access (CDMA), hybrids such asGSM, etc. which allow a single resource, should as a radio channel, tobe shared amongst multiple users. One common use for multiple accesssystems with radio channels is mobile telephone systems wherein multiplehandsets share the radio resources of a base station.

The ability of a radio receiver employing multiple access techniques tocorrectly receive a signal transmitted to it is generally limited by thesignal to noise ratio (SNR) the receiver experiences. The SNR experienceat a receiver is the ratio of the desired received signal to all othernoise sources, including thermal noise, radio noise (noise fromelectrical devices much as motors, etc.), transmissions from adjacenttransmitters (such as adjacent cells or sectors in a mobile telephonesystem) and other, non-orthogonal, signals transmitted from thetransmitter to which the receiver is listening.

As used herein the term “orthogonal signal” is intended to include allsignals which are avenged at transmission to have cross correlationsthat are ideally zero, or very small, e.g. CDMA signals are madeorthogonal via application of Walsh Codes, TDMA signals are madeorthogonal via assignment of time slots, etc. It should be noted that anorthogonal signal can be received at a receiver with its orthogonalitysomewhat reduced, due to multipath and other effects.

Clearly, the better the SNR experienced at a receiver, the bettor theability of the receiver to correctly receive the signal and the betterthe theoretical capacity of the system. as will be discussed furtherbelow.

One example of a widely used multiple access technique is code divisionmultiple access (CDMA), and specifically the direct sequenceimplementation of CDMA, which has recently gained significant support asthe multiple access technique of choice for advanced wirelesscommunication systems, such as mobile telephones or wireless local loopsystems. As is known, CDMA can offer advantages over many other multipleaccess techniques, in that planning and management of the network isgenerally simplified, with the guard bands or guard times typicallyrequired in FDMA or TDMA systems, for example, not being required andgood frequency reuse being obtained relatively easily.

As mentioned above, increases to the SNR experienced at a CDMA receiverare advantageous. Specifically, as the SNR experienced by a CDMAreceiver is increased more efficient use can be made of the CDMA codespace, with modulation orders being increased (for example from QPSK toQAM 16) and/or higher rate error correcting codes can be used (forexample increasing the code rate from ⅓ rate to ⅔ rate). As CDMA codespace is a limiting factor in the capacity of a CDMA communicationssystem, it is always desired to make efficient use of the code space.

Further advantages are obtained when transmissions in CDMA are performedat the lowest power level which is sufficient to provide the minimum SNRrequired for reception of the signal at the receiver at acceptable errorrace. By broadcasting to a first receiver at this minimum power level orvery close to it, the interference (noise) experienced at otherreceivers can be reduced, further increasing the efficiency and capacityof the CDMA system as the SNR's of those other receivers will beimproved.

Other multiple access systems benefit from improved SNR's in mannerssimilar to those of CDMA and, generally, an increase in the SNR ofsignals received at a receiver results in improved capacity and/orreliability of the communications system.

Accordingly, it is desired to have a system, method and apparatus whichcan allow a multiple seem communications receiver a improve the SNR ofdesired signals it otherwise receives from a transmitter, thus providingfor overall improved performance of the system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel system,method and apparatus for improving reception is a multiple accesscommunication system which obviates or mitigates at least some of theabove-identified disadvantages of the prior art. It is a further objectof the present invention to provide an improved multiple accesscommunications system and a method of operating such a system.

According to a first aspect of the present invention, there is provideda method of improving reception in a multiple access communicationssystem, comprising the steps of:

-   -   (i) determining at least one interfering signal transmitted from        a transmitter;    -   (ii) determining the received power level at a receiver of said        at least one determined interfering signal;    -   (iii) subtracting said at least one determined interfering        signal, at said received power level, from the total signal        received at said receiver, and    -   (iv) determining a desired signal from the result of said        subtraction.

According to another aspect of the present invention, there is provideda multiple access communication system including a plurality ofsubscriber stations and at least one base station to transmit signals tosaid subscriber stations, said subscriber stations comprising:

-   -   means to receive said signals transmitted by said at least one        base station;    -   means to determine at least one interfering signal transmitted        by said base station and the received power level of said at        least one interfering signal;    -   means to subtract said determined at least one interfering        signal at said received power level from said received signals;        and    -   means to determine a desired signal from the result of said        subtraction.

The present invention provides an apparatus, system and method forimproving reception in a multiple access telecommunications system bydetermining and subtracting known, or knowable, interfering signals fromthe total signal received at a subscriber station to obtain desiredsignals. Common channel signals, such as primary and secondarysynchronization signals, or any other known or knowable signals whichact as interference to a desired signal can be subtracted from thesignals received from a transmitter serving the receiver. Suchinterfering known or knowable signals received from other transmitters,such as adjacent sectors (in multi-sector wireless network systems) oradjacent base stations (to wireless networks) can also be subtracted.Further, known or knowable signals which are effectively interferingcommon channels to the receiver, such as orthogonal channels of anadjacent base station or sector, can also be subtracted.

In a 3GPP-type system, a receiver can subtract the primary and secondarysynchronization signals of the transmitter serving it, as well as thesame signals and the pilot channel signals of one or more adjacentsectors and/or base stations. In an IS-95-type system, a receiver cansubtract common channels of adjacent base stations and sectors. In othersystems, a receiver will be able to subtract known or knowableinterfering signals of transmitters.

By subtracting the known or knowable signals, the interferenceexperienced at a receiver is reduced allowing the power of the signaltransmitted to that receiver to be reduced while maintaining the sameSNR levels at the receiver. Thus, the level of interference experiencedat other receivers is also reduced, improving their experienced SNR andallowing signals transmitted to those receivers to also be transmittedat a reduced power level. Thus, the performance of a multiple accesscommunication system is enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

FIG. 1 shows a multiple access communications system;

FIG. 2 shows a schematic representation of an orthogonal signal and aninterfering signal broadcast and received at a receiver;

FIG. 3 shows the subtraction of known interfering signals from thesignal received at a receiver is accordance with the present invention;and

FIG. 4 shows a multiple access communications system with multi-sectoredbase stations.

DETAILED DESCRIPTION OF THE INVENTION

While the following discussion concentrates primarily on direct sequenceCOMA as a multiple access technique, and more specifically on theproposed 3GPP implementation, the IS-95 standard and the like, thepresent invention is not so limited and can be usefully employed withany receiver receiving multiple access signals wherein a received known,or knowable, signal that is interfering with a received desired signalcan be subtracted from the total received signal to improve thereception of the desired signal.

A multiple access communication system is indicated generally at 20 inFIG. 1. System 20 includes a plurality of transceivers, such as basestations 24, which are connected via suitable backhauls (not shown) andgateways (not shown) or the like to a public switched telephone network(PSTN) (not shown), other base stations 24, packet data networks such asthe Internet (not shown) and/or any other network of interest. Asmentioned above, system 20 employs a suitable multiple access techniqueand specifically, in this example, employs DS-CDMA.

The transmission range of each base station 24 defines a cell 28 withinwhich it can communicate with a plurality of transceivers such assubscriber stations 32. A subscriber station 32 can be a mobiletelephony device, a mobile data device or a wireless local loop device(providing telephony and/or data services). A cell 28 can include onlyone type of subscriber station 32 (such as mobile voice telephonydevices) or a variety of types of subscriber stations 32 (such as mobilevoice telephony and fixed voice and/or data devices). At any one time, asubscriber station 32 is typically served by the base station 24 fromwhich it can receive signals with the best signal to noise ratio (SNR).

Data transmitted from a base station 24 to a subscriber station 32 istypically encoded with an error correcting code, such as a convolutionalcode, and this is typically described in terms of the resultingeffective information rate. Specifically, transmissions are described asbeing coded at ⅓, ½, ⅔, ¾, etc. rates (i.e.—at ½ rate, two bits of dataare transmitted for every one actual information bit, at ¾ rate, fourbits of data are transmitted for every three actual information bits).As is known to those of skill in the art, by employing “puncturing”,wherein a coded signal has a defined number of its coded bits replacedwith additional data bits, a fine granularity of code rates can beachieved.

When low SNR's are expected at a subscriber station 32, low coding rates(i.e. —¼) are employed to ensure acceptable bit and/or frame error ratesare experienced. As the SNR experienced at a subscriber station 32improves, higher rate codes can be employed (e.g. —changing from a ¼code to a ½ code) with a resulting improvement in the efficiency atwhich the resources (transmission capacity, or bandwidth) of system 20are used.

Similarly, when relatively low SNR's are experienced at a subscriberstation 32, transmitted data is typically modulated using low orderschemes, such as QPSK, to improve the ability of the subscriber station32 to reliably receive the transmitted data. As the SNR experienced atthe subscriber station 32 improves, mote aggressive (higher order)modulation schemes, such as QAM 16, QAM 64 or QAM 256 modulations can beemployed, again resulting in an improvement of the efficiency at whichthe resources (transmission capacity, or bandwidth) of system 20 areused.

In other circumstances, if the SNR of the signal received at asubscriber station 32 improves, the power at which that signal istransmitted can be decreased until the SNR just exceeds the level neededto reliably receive the signal at a given code rate and/or modulationorder and thus the degree to which the signal interferes with receptionof signals at other subscriber stations 32 can be reduced, allowingthose subscriber stations 32 to obtain an improved SNR.

The combinations of error coding and modulation schemes employed can beselected for a given SNR level to achieve a desired probability of frameerror rate for transmissions while making efficient use of the resourcesof system 20. Clearly, any improvement in the SNR experienced at areceiver can provide an improvement in system efficiency and/orreliability.

In modern CDMA systems and other multiple access systems, channelschemes are employed to organize and permit acquisition, setup and useof communication between base stations 24 and subscriber stations 32.Typically, some of these channels are common channels, i.e.—arebroadcast to all subscriber stations 32 in a cell 28, and other channelsare orthogonal channels. As is discussed below, in some cases, commonchannels act as interference to other signals broadcast to subscriberstations 32 and orthogonal channels from a transmitter can act asinterference to subscriber stations 32 receiving signals from othertransmitters.

System 20 can be a system constructed in accordance with standards suchas IS-95, wCDMA, CDMA2000, the proposed 3GIPP system standard presentlybeing agreed, or any system employing a channel scheme with one or morecommon channels or other interfering channels over which known orknowable data is transmitted. While the following discussion refers tothe above-mentioned proposed 3GPP standard, the present invention is notlimited to use with this system and can also be used other standards,such as the IS-95, wCDMA, CDMA200 or other channel schemes, as will beapparent to those of skill in the art.

As used herein, the term “knowable” signal is intended to comprise anysignals which can be determined by a receiver by any means. Examplesinclude predefined control signals, such as synchronization signals,scrambled signals, which can be descrambled by a receiver once adescrambling code is known, signals which can be determined afterexplicit or blind detection operations, etc.

In the proposed 3GPP system, the common channels broadcast in thedownlink direction (i.e.—from base station 24 to subscriber stations 32)include a primary synchronization channel (PSCH) and a secondarysynchronisation channel (SSCH). In the 3GPP system, many channels(including the orthogonal channels) are broadcast in frames of 10 msduration, wherein each frame includes fifteen slots.

As discussed in the 3GPP documentation, which is publicly available froma number of sources (including the 3GPP organization website atwww.3GPP.org), the PSCH is used by subscriber stations 32 to determinethe timing of slots within frames transmitted by a base station 24. Apredefined, known, data sequence is transmitted to the slots and framesof the PSCH and this sequence has been defined and arranged such thatsubscriber stations 32 can determine the start time of slots in framestransmitted by the base station 32.

Once the slot timing has been determined by a subscriber station 32 fromthe PSCH, the SSCH is examined by the subscriber station 32 to determinethe timing of the frames of the slots and other information, includingscrambling codes used by the base station 24, etc. The acquisition andprocessing of the PSCH and SSCH channels is performed at start up of asubscriber station 32 within system 20 and, in mobile systems at least,is performed on an ongoing basis for adjacent base stations 24 to permithandoffs between cells 28.

While such a system does result in a flexible and robust communicationsystem, it suffers from disadvantages in that the common channels act asinterference at the subscriber stations 32 in the cell 28 with respectto the other received signals, reducing thus SNR experienced at thesubscriber stations 32.

FIG. 2 shows a simplified representation of the transmission to asubscriber station 32 of a bit of a desired signal “D₁”. As received atsubscriber station 32, D₁ will be combined with the PSCH, SSCH, etc.FIG. 2 shows the simplified case of the combination of D₁ with just thePSCH for clarity. When D₁ is transmitted from base station 24, D₁ isspread by multiplying it with the ‘chips’ c₁ through c_(i) of apreselected spreading code. This results in the product “D₁c₁, D₁c₂,D₁c₃ . . . D₁c_(i)” which has some amount of gain performed on it byamplifier 90. The PSCH, which comprises a predefined series of chips s₁through s_(i), has some amount of gain performed on it by amplifier 94and is added to the above-mentioned product, at summer 98, to obtain theresult “D₁s₁, D₁s₂, D₁s₃ . . . D₁s_(i)”. This result is broadcast to asubscriber station 32 over radio link 102. Some amount of noise “n” isinevitably added to the result received at subscriber station 32 duringthe radio broadcast, as represented by the summer in radio link 102.

At the receiver of the subscriber station 32, the received signal is ofthe form “D₁c₁+s₁+n, D₁c₂+s₂+n, D₁c₃+s₃+n, . . . D₁c_(i)+s_(i)+n” and adot product operation 106 is performed on this result to obtain“D₁(c₁c₁+c₂c₂+c₃c₃+. . .+c_(i)c_(i))+(c₁s₁+c₂s₂+c₃s₃+. . .c_(i)s_(i))+n”, where the effect of noise n has lumped together andrepresented as a single value, n. As it is known that the values of thechips “c” can only be −1 or +1 and the values of primary synchronizationchips “s” can only be −1 or +1, this result can be simplified to “4iD1+(c₁s₁+c₂s₂+c₃s₃+ . . . c_(i)s_(i))+n”. In conventional communicationsystems, an estimate is then performed at the receiver by a suitablemeans, a viterbi decoder for example, on this result to determine thevalue of D₁. The process repeats for the next bit, D₂, of the desiredsignal.

While this system has been successfully employed in the past, thepresent inventor has realized that the term “(c₁s₁+c₂s₂+c₃s₃+. . .c_(i)s_(i))+n” in the result is, in fact, interference to the desiredsignal D₁, thus reducing the overall SNR experienced at a receiver andthat similar interference will also occur from the SSCH signal. In fact,the interference term defined above for the PSCH alone can be written“(c₁sp₁+c₂sp₂+c₃sp₃+. . . c_(i)sp_(i))+(c₁ss₁+c₂ss₂+c₃ss₃+. . .c_(i)ss_(i))+n”, where sp_(i) represents the PSCH signal and ss_(i)represents the SSCH signal.

The present inventor has determined that the SNR experienced at asubscriber station 32 can be improved by subtracting the known PSCH andSSCH signals from the signals received at the subscriber station 32.Specifically, by receiving the PSCH and SSCH to determine the timing ofthe frames end slots transmitted by base station 24, subscriber station32 has also determined the power at which the PSCH and SSCH signals havebeen received at the subscriber station 32. Therefore, once acquisitionhas been achieved and a subscriber station 32 is ready to operate, thesubscriber station 32 knows the PSCH and SSCH data it has received andthe power level they were received at. In accordance with the presentinvention, the receiver in subscriber station 32 then subtracts theseknown signals from the total signal received at the subscriber station32 to reduce this source of interference to other received signals asshown in FIG. 3. Thus, the term “(c₁sp₁+c₂sp₂+c₃sp₃+. . .c_(i)sp_(i))+(c₁ss₁+c₂ss₂+c₃ss₃+. . . c_(i)ss_(i))+n” can be reduced to“n” as the values of each c_(i) and sp_(i) (primary synch) and ss_(i)(secondary synch) will be known at the receives. Each subsequent databit D_(i) is processed in a similar manner, with the transmission powerlevels of the PSCH and the SSCH being updated accordingly before beingsubtracted from the total received signal.

It is known that as much as 20% or more of the total power transmittedby a base station 24 will typically be utilized to transmit the PSCH andSSCH channels. As will be apparent, subtracting the PSCH and SSCHsignals from the total received signal at a subscriber station 32 canresult in a corresponding improvement to the SNR of the subscriberstation 32.

While the subtraction is discussed above as being performed at thesymbol level, in some circumstances it will be preferred that thesubtraction be performed at the chip level and appropriate methods ofimplementing such will new be apparent to those of skill in the art. Byperforming the subtraction at the chip level, data signal bits D_(i),etc. which are transmitted at different symbols rates, but the seinechip rate, can be appropriately processed.

When a subscriber station 32 performs the subtraction of known signalsin accordance with the present invention, system 20 can be operated in amanner which benefits from the effective decrease in signals received atsubscriber station 32. Specifically, system 20 can decrease the poweremployed to transmit desired signal bit D_(i) to subscriber station 32,can employ a higher rate coding scheme when transmitting D_(i) and/orcan employ a higher order modulation scheme to transmit D_(i). Theseoptions can be used individually or combined, as appropriate or desired,to improve the transmission capacity of system 20. Further, subscriberstations 32 which are at the fringes of cell 28, and thus have marginalreception ability, can improve their SNRs, reducing their error rates orincreasing their reception data rates, even though base station 24cannot allocate additional power to their transmitted signals, due toregulatory restrictions or transmitter capabilities, and/or when nopractical lower coding rate is available and/or when no lower ordermodulations are available. Thus it is possible to obtain improvements intotal transmission capacity of base stations 24 in system 20 and/or thetransmission footprint (cell size) of the base stations 24.

The present invention is not limited to the reduction of interference bythe subtraction of synchronization channels and, in fact, anyinterfering signal which is known or knowable, such as any othercommunication system control signal or information signal, can besubtracted firm the total signal received at a receiver to improvereception of a desired signal.

The present inventor has also developed a second embodiment of thepresent invention which can further improve the performance of system 20in some circumstances. One of the limiting factors of the performance inCDMA systems, or in other multiple access systems, is interference frombase stations 24 in adjacent cells 28. As shown in FIG. 1, a cell 28 canbe considered as the geographic area which can be reliably serviced by abase station 24. While cells 28 are commonly illustrated as havingregular shapes, commonly hexagonal or circular (as illustrated in FIG.1), typically a cell 28 does not have a regular shape due togeographical features, such as hills, valleys, buildings, bridges, etc.,or other conditions which affect the distance the radio signals from abase station 24 can reliably propagate.

Due to the irregular shape of cells 28, it is not uncommon that a cell28 includes areas 36 that overlap with an adjacent cell 28. Such areasof overlap can also be formed intentionally by network planners to allowfor handoff (transfer of a subscriber station 32 from one cell toanother) or to provide additional capacity in “hot spots” in a network.In the illustrated configuration of system 20 in FIG. 1, four areas ofoverlap (36 ab, 36 ac, 36 bc, and 36 abc) are shown between calls 28 a,28 b and 28 e.

While a subscriber station, such as subscriber station 32 ₁, can beserviced from other base station 24 a or 24 c (in this example 24 a)downlink transmissions from the other base station (24 c in thisexample) appear to subscriber station 32 ₁ as interference to thedownlink transmissions from base station 24 a. Any subscriber station32, such as subscriber station 32 ₁ which receives the downlinktransmission of more than one base station 24 will experience areduction in its SNR with respect to the downlink transmissions receivedfrom the base station 24 that is presently servicing it. In fact, evensubscriber stations, such as station 32 ₂, that are outside of overlaps36 where another base station could be reliably received, willexperience some interference from those other base stations 24 althoughat low received power levels, thus reducing their SNR. Some othersubscriber stations, such as subscriber stations 32 ₃ and 32 ₄, do notreceive significant levels of interference from other base stations 24as they are located well outside of the effective propagation area ofsuch adjacent base stations 24. However, in a system 20 with multiplecells 28, there is typically only a relatively small number ofsubscriber stations 32 which do not receive interference from adjacentbase stations 24 that measurably degrades their SNRs.

Accordingly, the present inventor has determined that a subscriberstation 32 can also improve its SNR by subtracting known or knowablechannels received from one or more adjacent base stations 24.Specifically, at start up, and typically on an ongoing basis, asubscriber station 32 performs an sequence of operations wherein itdetermines the base station 24 that it can best receive (based upon thebest SNR). In this second embodiment of the present invention, asubscriber station, such as station 32 ₁, can re-perform thisacquisition sequence of operations to also determine the base station 24c which it can receive at the next-best levels (typically, the basestation receiver at the next highest received power level). Ifsubscriber station 32 ₁ determines that this next-best base station 24 cis being received at a power level which is higher than a pre-definedminimum power level, it will proceed to subtract the signals that itknows, which are received from the next-best base station 24 c, from thetotal signal it receives to improve the SNR of the base station 24 a itis being serviced by as described below.

If the next-best base station 24 c is received at a power level lessthan the pre-defined level, it is deemed that the potential benefit ofsubtracting the signals received from it is not sufficient to justifyperforming these step, and no determination and subtraction of thesesignals will be performed. As will be apparent, this determination of anext-best received base station 24 will be performed, from time to time,to ensure that as reception conditions chance over time an appropriateaction is taken.

If next-best base station 24 c is received at a power level greater thanthe pre-defined minimum, the subscriber station 32 ₁ determines the slotand frame timing and the power levels of the transmissions fromnext-best base station 24 c, using the same techniques as before, andsubtracts the PSCH and SSCH of next-best base station 24 c in a mannersimilar to that described above. Specifically, subscriber station 32_(i) will receive a signal “4 iD1+(c₁₁sp₁₁+c₁₂sp₁₂+c₁₃sp₁₃+. . .c_(1i)sp_(1i))+(c₁₁ss₁₁+c₁₂ss₁₂+c₁₃ss₁₃+. . .c_(1i)ss_(1i))+(c₂₁sp₂₁+c₂₂sp₂₂+c₂₃sp₂₃+. . .c_(2i)sp_(2i))+(c₂₁ss₂₁+c₂₂ss₂₂+c₂₃ss₂₃+. . . c_(2i)ss_(2i))+n”, wherec_(1i), sp_(1i) and ss_(1i) are the chip, primary and secondary synchsignals of the base station 24 a and c_(2i), sp_(2i) and ss_(2i) are thechip, primary and secondary synch signals of next-best base station 24c. A dot product is performed on this received signal and the terms“(c₁₁sp₁₁+c₁₂sp₁₂+c₁₃sp₁₃+. . . c_(1i)sp_(1i))”,“(c₁₁ss₁₁+c₁₂ss₁₂+c₁₃ss₁₃+. . . c_(1i)ss_(1i))”,“(c₂₁sp₂₁+c₂₂sp₂₂+c₂₃sp₂₃+. . . c_(2i)sp_(2i))” and“(c₂₁ss₂₁+c₂₂ss₂₂+c₂₃ss₂₃+. . . c_(2i)ss_(2i)) are subtracted to obtain4 iD₁+n, from which the desired signal D₁ can be determined. Similaroperations can be performed for other multiple access systems, such asIS-95 or other wireless systems.

in addition to known interfering common channels, channels which arebroadcast as orthogonal channels by an adjacent base station 24 c, suchas base station 24 c, also serve as interference to subscriber stations32 which are being served by a first base station 24, such as basestation 24 a, either because they use a scrambling code and/or Walshcode which is used by the first base station and/or because the timingof their transmission is not synchronized with that of the first basestation. If the information transmitted is an orthogonal, butinterfering, channel from another base station 24 is known, or can bedetermined (i.e.—is knowable), by a subscriber station 32, thatorthogonal channel can also be subtracted from the total received signalat the subscriber station 32, further improving the SNR of the desiredsignals received at the subscriber station 32.

As a specific example, in systems adhering to the proposed 3GPPstandard, at least one orthogonal channel at an adjacent base stationwill be known or knowable by a subscriber station 32. Specifically, apilot channel (PICH) is transmitted by each base station 24 and is usedfor carrier offset determination at subscriber stations 32 and for otherpurposes. The pilot signal comprises a series of 1's that are scrambledaccording to the scrambling code of the transmitting base station 24 andare transmitted over a pre-defined channel.

Accordingly, after a subscriber station 32, such as subscriber station32 ₁ has determined that it is receiving interference from an adjacentbase station, for example 24 c, at a power level above the pre-definedlevel, it can determine the scrambling code group of this next-best basestation 24 c, determine the annul scrambling code from the otherinformation transmitted by the next-best base station 24 c, in the samemanner that a subscriber station 32 served by that base station 24 cwould determine the scrambling code, and determine the signal that wouldresult from the scrambled series of “1's”. Alternatively, the scramblingcode can be directly communicated to the subscriber station 32 in avariety of manner including via a transmission from the base station 24a serving it which can transmit the scrambling codes of all adjacentbase stations 24. Thus, knowing the scrambling code and the data (theseries of 1's), the knowable PICH is now known to the subscriberstation.

At this point, the subscriber station 32 ₁ then determines the powerlevel at which the PICH is received as this can change over time. It ispresently contemplated that an indication of the transmission powerlevel of the PICH will be provided by the transmitting next-best basestation 24 c, over the BCCH or other suitable channel, although anyother suitable means as will occur to those of skill in the art can beemployed, such as by estimation means, back-haul based communicationbetween base stations 24 and wherein the power levels of all adjacentbase station PICH transmissions are transmitted to subscriber stations32 served by a base station 24, etc. It is also contemplated that thePICH power level will be provided to subscriber stations 32 as atransmission power level which is expressed relative to the transmitpower of the PSCH and/or SSCH of the next-best base station 24, whosereceived power levels are determined by the subscriber unit 32 as partof the above-mentioned acquisition process. Thus, the received powerlevel of the PICH can be determined by subscriber stations 32. At thispoint, the subscriber station 32 ₁ “knows” the PSCH, SSCH and PICH ofthe next-best received base station 24 c and can subtract those signalsfrom the total received signal at the subscriber station 32 ₁ to improvethe SNR of the signals it receives from the base station 24 a from whichit is being served.

It is expected that as much as 45% or more of the total powertransmitted by a base station 24 will typically be utilized to transmitthe PSCH, SSCH and PICH channels. As will be apparent, subtracting thesesignals from the total received signal at a subscriber station 32 canresult is a corresponding improvement to the SNR of the subscriberstation 32.

FIG. 4 shows another configuration of a multiple access communicationsystem 200 wherein one or more base station 240 employ beam formingantennas (not shown), or other means, to divide their cells 248 intodifferent sub-cells, typically referred to as sectors 260. Each sector260 of a base station 240 communicates with the subscriber station 32within its beam path and range and each sector 260 in abase station 240is provided with a transceiver for such communications. FIG. 4illustrates cells 248 a, 248 b and 248 c as each having six sectors 260ax, 260 b _(x) and 260 _(c) _(x) respectively. As will be apparent tothose of skill in the art, cells 248 can have different numbers ofsectors 260 and system 200 can include either a homogeneous set of cells248, each with the same number of sectors 260, or can include aheterogeneous set of cells 248 some of which have different numbers ofsectors 260, e.g.—some cells 248 with a single sector 260, some cells248 with two sectors 260, etc.

In these sectored configurations, which are expected to be commonlydeployed, a subscriber station, such as subscriber station 32 ₈ insector 260 a ₂ can, and often will, receive transmissions intended foras adjacent sector, such as 260 a ₃ or 260 a _(i). Such transmissionsact as interference at subscriber station 32 ₈ with respect to thetransmissions it is attempting to receive from sector 260 a ₂.Accordingly, subscriber station 32 ₈ can perform similar operations tothose described above for reducing interference from adjacent basestations to reduce interference from adjacent sectors 260. Dependingupon the configuration of system 200, sectors 260 within a cell 248 cantransmit frames and slots in a synchronous manner (with time offset orwithout) to the other sectors 260 of cell 248 or asynchronously.

In the synchronous case, a subscriber station 32 will the signal on thePSCH of each other sector 260 in its cell 248, the signal on the PSCHeither being identical in each sector 260, or being time offset from thesignal on the PSCH in the sector 260 of the subscriber station 32 by atime (usually by a slot, or multiple thereof, in a frame) known to thesubscriber station 32. Thus, in this case, the subscriber station 32 caneasily subtract the PSCH of adjacent sectors 260.

In systems constructed in accordance with the proposed 3GPP standard,each sector 260 will have its own scrambling code. Thus, as with theembodiments described above, the SSCH in each sector 260 transmits thescrambling code group for that sector 260 and this scrambling code mustbe determined so that the SSCH and, as discussed below, the PICH can besubtracted. As sectors 260 within a cell 248 operate within a singlebase station 240, base station 240 can inform subscriber stations 32 ineach sector 260 of cell 248 of the relevant scrambling code group andscrambling code by any suitable means, such as by transmission throughthe BCCH. Alternatively, the subscriber station 32 can determine thescrambling code group and scrambling code during an acquisition processfor an adjacent sector, as described above for the adjacent base stationcase.

In the asynchronous case, a subscriber station 32 can treat an adjacentsector 260 is the same manner as that described above for an adjacentnext-best base station 24 and can derive the frame and slot timing fromthe PSCH and SSCH, etc.

As will be apparent to those of skill in the art, the above-describedembodiments to be combined as required. For example, a subscriberstation 32 can subtract the PSCH and SSCH signals from the base station(or sector) serving it and can determine whether an adjacent sector 260or adjacent base station 24 is the highest other source of interferencefor it and can adopt the appropriate strategy, from those describedabove, to reduce this additional interference. Further, it will beapparent to those of skill in the art that the present invention is notlimited to canceling only the interference from one adjacent next bestbase station or sector and, if radio and computational resources areavailable. In subscriber station 32, subtraction of known or knowablesignals from two or more adjacent base stations and/or sectors can beperformed in addition to the subtraction of the PSCH and SSCH from thebase station or sector swing the subscriber station 32.

The present invention provides an apparatus, system and method forperformance to the downlink direction of a CDMA telecommunicationssystem. Known, or knowable, signals are determined and subtracted fromthe signals received at a subscriber station, whether a mobile or fixedstation and whether the signal is voice or data or both. Subtraction ofthese signals improves SNR for the desired signals at the subscriberstation 32, which allows transmission of the desired signals in theseefficient moment, i.e.—with higher (¾ vs. ½, etc.) error coding ratesand/or increased modulation orders (QAM64 vs QPSK), and/or allows thedesired signals to be broadcast at a lower power level while beingreceived with the same, or similar, error levels.

By subtracting the known signals. Including those determined fromknowable signals, the interference experienced at a subscriber stationis reduced allowing the power of the signal transmitted to thatsubscriber station to be reduced while maintaining the same SNR levelsat the subscriber station. Thus, the level of interference experiencedat other subscriber stations is also reduced, improving theirexperienced SNR and allowing signals transmitted to those subscriberstations to also be transmitted at a reduced power level, higher ratecode or higher order modulations. Thus, the performance of a multipleaccess communication system is enhanced.

The above-described embodiments of the invention am intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined, solely by the claimsappended hereto.

1. A method of improving reception in a multiple access communicationssystem, comprising the steps of: (i) determining at least oneinterfering signal transmitted from a transmitter; (ii) determining thereceived power level at a receiver of said at least one determinedinterfering signal; (iii) determining at least one interfering signaltransmitted from another transmitter; (iv) determining the receivedpower level at said receiver of said at least one determined interferingsignal from said another transmitter; (v) subtracting said at least onedetermined interfering signal at step (i) at said received power leveldetermined at step (ii), from a total signal received at said receiver;(vi) comparing the received power level determined in step (iv) to apredefined threshold level and omitting step (vii) when said thresholdis not exceeded; (vii) subtracting said at least one determinedinterfering signal at step (iii) at said received power level determinedat step (iv) from said total signal received at said receiver; and (vii)determining a desired signal from the result of said subtractions. 2.The method of claim 1 wherein said at least one interfering signalcomprises a synchronization signal.
 3. The method of claim 1 wherein atleast two interfering signals are transmitted by said transmitter andsaid receiver determines each of said at least two interfering signalsand their respective received power levels and subtracts thosedetermined interfering signals at their respective received power levelsfrom said total received signal.
 4. The method of claim 3 wherein saidat least two interfering signals comprise a first synchronization signalfor determining slot timing in signals transmitted by said transmitterand a second synchronization signal for determining frame timing insignals transmitted by said transmitter.
 5. The method of claim 1wherein said at least one interfering signal is a communication systemcontrol signal.
 6. The method of claim 1 wherein said interfering signaldetermined in step (iii) comprises a non-interfering signal to at leastone other receiver.
 7. The method of claim 6 wherein said interferingsignal determined in step (iii) comprises a pilot signal.
 8. The methodof claim 1 wherein said other transmitter comprises an adjacent basestation.
 9. The method of claim 1 wherein said other transmittercomprises an adjacent sector of a multi-sector base station.
 10. Themethod of claim 1 wherein the step of comparing is performed atpredefined intervals.
 11. A method of improving reception in a multipleaccess communications system, comprising the steps of: (i) determiningat least one interfering signal transmitted from a transmitter; (ii)determining the received power level at a receiver of said at least onedetermined interfering signal; (iii) determining at least oneinterfering signal transmitted from another transmitter; (iv)determining the received power level at said receiver of said at leastone determined interfering signal from said another transmitter; (v)subtracting said at least one determined interfering signal at step (i)at said received power level determined at step (ii), from a totalsignal received at said receiver; (vi) subtracting said at least onedetermined interfering signal at step (iii) at said received power leveldetermined at step (iv) from said total signal received at saidreceiver; and (vii) determining a desired signal from the result of saidsubtractions, wherein steps (iii) and (iv) are performed to select, fromat least two other transmitters, the transmitter with the highestreceived power level in step (iv) and step (vi) is performed for saidselected other transmitter.
 12. The method of claim 11 wherein said atleast one interfering signal comprises a synchronization signal.
 13. Themethod of claim 11 wherein at least two interfering signals aretransmitted by said transmitter and said receiver determines each ofsaid at least two interfering signals and their respective receivedpower levels and subtracts those determined interfering signals at theirrespective received power levels from said total received signal. 14.The method of claim 13 wherein said at least two interfering signalscomprise a first synchronization signal for determining slot timing insignals transmitted by said transmitter and a second synchronizationsignal for determining frame timing in signals transmitted by saidtransmitter.
 15. The method of claim 11 wherein said at least oneinterfering signal comprises a communication system control signal. 16.The method of claim 11 wherein said interfering signal determined instep (iii) comprises a non-interfering signal to at least one otherreceiver.
 17. The method of claim 16 wherein said interfering signaldetermined in step (iii) comprises a pilot signal.
 18. The method ofclaim 11 wherein said other transmitter comprises an adjacent basestation.
 19. The method of claim 11 wherein said other transmittercomprises an adjacent sector of a multi-sector base station.
 20. Themethod of claim 11 wherein steps (iii) and (iv) are performed atpredefined intervals to select, from at least two other transmitters,the transmitter with the highest received power level in step (iv), andstep (vi) is performed for said selected other transmitter.
 21. A methodof improving reception in a multiple access communications systemcomprising the steps of: (i) determining the received power level at areceiver of at least one interfering signal transmitted from atransmitter, wherein said at least one interfering signal ispredetermined; (ii) subtracting said at least one interfering signal, atsaid received power level, from a total signal received at saidreceiver: (iii) determining a desired signal from the result of saidsubtraction; iv) determining the received power level at said receiverof at least one interfering signal from another transmitter, whereinsaid at least one interfering signal from another transmitter is known apriori; (v) performing step (ii) by also subtracting said at least oneinterfering signal from another transmitter at the received power leveldetermined at step (iv) from said total signal received at saidreceiver; (vi) performing step (iii) to determine a desired signal fromthe result of the subtractions; and vii comparing the received powerlevel determined in step (iv) to a predefined threshold level andomitting steps (v) and (vi) when said threshold is not exceeded.
 22. Themethod of claim 21 wherein the step of comparing is performed atpredefined intervals.
 23. A method of improving reception in a multipleaccess communications system, comprising the steps of: (i) determiningthe received power level at a receiver of at least one interferingsignal transmitted from a transmitter, wherein said at least oneinterfering signal is predetermined; (ii) subtracting said at least oneinterfering signal, at said received power level, from a total signalreceived at said receiver; (iii) determining a desired signal from theresult of said subtraction; (iv) determining the received power level atsaid receiver of at least one interfering signal from anothertransmitter, herein said at least one interfering signal from anothertransmitter is known a priori; (v) performing step (ii) by alsosubtracting said at least one interfering signal from anothertransmitter at the received power level determined at step (iv) fromsaid total signal received at said receiver; and (vi) performing step(iii) to determine a desired signal from the result of the subtractions,wherein step (iv) is performed to select, from at least two othertransmitters, the transmitter with the highest received power level instep (iv) and steps (v) and (vi) are performed for said selected othertransmitter.
 24. The method of claim 23 wherein step iv is performed atpredefined intervals to select the transmitter with the highest receivedpower level and steps (v) and (vi) are performed for said selected othertransmitter.